If you're watching different films (on different TV sets) then you'll need to use each transmitter/headphones pairing as normal, while ensuring that they're on different channels.
If you're watching the same TV programme (on the same TV) then (as long as they're the same make, and thus using the same channels) you can discard one of the transmitter units (other than for charging purposes) and just plug one transmitter into the TV's headphone socket (with both sets of headphones switched to the same channel).